(1) Field of the Invention
The present invention relates to the fabrication of integrated circuit devices on semiconductor substrates, and more particularly relates to a method for making bipolar and MOS devices using variations in the punchthrough and field oxide (FOX) thickness formed by the LOCal Oxidation of Silicon (LOCOS) method. The multiple thickness of the LOCOS is used as a self-aligning implant mask, and to electrically isolate the device areas. This eliminates the multiphotoresist masking steps for ion implantation and the accompanying alignment tolerance limitations, thereby providing high device-packing density and a cost-effective manufacturing process.
(2) Description of the Prior Art
Typically the fabrication of bipolar and MOS transistors requires a considerable number of processing steps. Because of their vertical structure, bipolar transistors require a series of separate implant masks and ion implant steps for making the emitter, base, and collector This is because the bipolar requires multiple masking and implant steps to form the collector, base, and emitter at different depths in the silicon substrate. On the other hand, the FETs, such as used on dynamic random access memory (DRAM) circuits are basically surface devices that are simpler to fabricate, and can have improved device characteristics by including a doped shield region under the FET source, which can increase driver current and increase breakdown voltage between source and drain. However, an additional masking step is required to implant the shield region under the FET source. Therefore, it is desirable to be able to fabricate the bipolar and/or field effect transistors with reduced complexity. It is also desirable to provide a single self-aligning implant mask to decrease device size and to increase device-packing density, which would not be achieved otherwise by the conventional method in which separate lithographic masking and alignment tolerances limit device size.
One method of using a screen oxide layer having various thicknesses to form devices at various depths in the substrate is described by Joyner, U.S. Pat. Nos. 5,364,800 and 5,548,149. Joyner teaches a method of patterning a screen oxide implant layer having different thickness which he then uses to implant oxygen to form silicon-on-insulator (SOI) substrates having silicon device areas of various depths.
Therefore, there is still a strong need in the semiconductor industry to make circuits with improved self-aligned semiconductor devices for improved density and circuit performance while reducing manufacturing cost.